In field-transfer-type CCD imagers present practice is to periodically transfer all the lines of charge descriptive of an image integrated over field times in an image (or A) register into a field storage (or B) register during intervening field retrace intervals. In each ensuing field trace interval the transferred charge is advanced in the field storage register a line at a time during line retrace interval. The charge packets descriptive of picture elements (pels) in the last line are transfered in parallel to a line-scanning (or C) register. During the following line trace interval the C register is clocked to supply the charge packets serially to its output stage for conversion to successive samples of CCD imager video signal output.
It is found to be desirable to shorten the time required for field transfer, without increasing the rate at which lines are clocked out of the image register. This is desirable for example when the A register has many lines of charge, as in a high-resolution imager, and the video signal standards restrict field retrace time. Shortening of field transfer time is, by way of further example, helpful in reducing transfer smear.
The phenomenon of transfer smear is described in U.S. Pat. No. 4,010,319 issued 1 Mar. 1977 to P. A. Levine and entitled SMEAR REDUCTION IN CCD IMAGERS, incorporated herein by reference. Transfer smear arises when the image register is exposed to radiation during the transfer of charge packets from image register to field storage register, because the photoresponse to the image at this time is misregistered with respect to the integrated photoresponses being transferred. The severity of transfer smear is known to be in direct proportion to the time taken for transfer of charges from image register to field storage register. However, limitations on the speed with which lines of charge can be clocked forward from image register to field storage register have hampered reduction of transfer smear in an unshuttered imager.
Levine teaches how to generate samples of smear response alone, to be differentially combined with samples of image response contaminated with smear for obtaining image response with reduced smear contamination. Electronic shutter schemes for back-illuminated CCD imagers, which employ a retarding field for photocarriers generated during field transfer are known. But these methods even in combination are only partially effective, and further reduction in field transfer times would help these methods provide acceptable smear reduction.